1. Field of the Invention
The field of the invention is fluidized catalytic cracking (FCC) of heavy hydrocarbon feeds and selective catalytic reduction (SCR) of nitrogen oxides from the FCC regenerator.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO.sub.2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Catalyst regeneration usually causes formation of NOx, either in the regenerator, if operating in full CO combustion mode or in a downstream CO boiler, if operating in partial CO combustion mode. NOx emissions are becoming more of a problem, as FCC units are being forced to process worse feeds containing more NOx precursors, and as environmental regulations become stricter.
There are many approaches towards operating the FCC unit to reduce NOx emission, various catalyst additives, segregated cracking of different feeds, and regenerator modifications. These are all helpful, but can only achieve a modest reduction in NOx emissions. Some refiners need to do more, and resort to flue gas treatments to remove NOx. There are two primary NOx flue gas treatments available, thermal and catalytic.
Thermal DENOx involves operation at 870.degree.-980.degree. C. with urea or ammonia addition to reduce NOx. Capital costs are moderately high, because of the high temperatures, and operating costs are higher than desired, again because a large volume gas stream must be heated. Thermal DENOx is preferred by many refiners for FCC use because it works with no catalyst. A drawback to this approach is that the maximum amount of NOx reduction achievable is typically about 50%.
Catalytic reduction of NOx, the SCR process, is a proven technology used to reduce NOx emission for many refinery processes. It operates at moderate temperatures, well below those of FCC regenerators, so capital and operating costs are moderate. It adds a roughly stoichiometric amount of ammonia to a NOx containing flue gas stream and relies on a catalyst, usually honeycomb monoliths, to promote the reduction of NOx by NH3. The process works best with flue gas from furnaces, which can have moderate amounts of NOx and other gaseous pollutants, but are relatively free of particulates.
SCR has never been too successful for use on cleaning up FCC flue gas stream because the catalyst fines invariably found in the FCC flue gas will overwhelm the SCR catalyst.
A typical FCC flue gas stream, even with a third stage separator, can have dust loadings above 100 mg/NM.sup.3. While third stage separators or a baghouse might reduce this to some extent, refiners usually must resort to an electrostatic precipitator to reduce dust loading below 50 mg/NM.sup.3. A good electrostatic precipitator might reduce dust loadings to the 10-30 mg/NM.sup.3, with 20 mg/NM.sup.3 being readily achievable. Such dust loadings are too high to permit long service life of the commercially available SCR catalyst elements with a honeycomb design. The very nature of the honeycomb catalyst elements, with many parallel, small diameter paths means that dust particles will not have to travel far to reach a catalyst surface, where they tend to stay and accumulate. The honeycomb elements reduce turbulence and induce a laminar flow regime through the parallel cells of the honeycomb catalyst.
The FCC catalyst fines do not react with the SCR catalyst elements, rather they blanket the catalyst with a layer of dust. The diffusion limitations introduced by the catalyst fines prevents the active and long lived catalyst elements from working effectively. Some of the attempts at dealing with catalyst fouling will now be reviewed.
First, it should be noted that most commercial NOx conversion technologies now use honeycomb or monolithic catalyst elements. These have much lower pressure drops than packed beds of catalyst, and gas can continue to flow even when large amounts of fines are present.
U.S. Pat. No. 4,867,953 taught SCR using a rotating basket or honeycomb element, Exhaust gas with NOx would flow over a segment, which would eventually rotate so that incoming air, flowing in the opposite direction, could displace some of the particulates from the catalyst.
U.S. Pat. No. 4,670,226 disclosed a moving bed reactor for treating a dust laden gas.
U.S. Pat. No. 4,246,234 taught reducing nitric oxide in a dust laden gas with ammonia using a reactor passing the gas near, but not through, the catalyst bed. The only way NOx reached catalyst was via diffusion from a plurality of centrally located tubes containing fine perforations. Dust laden gas passed through the tubes, but never impinged directly on catalyst.
U.S. Pat. No. 4,682,470 Shaff disclosed a catalytic converter for cars, with a bed of catalyst beads under a compressive force, so that the catalyst would not move around.
U.S. Pat. No. 3,733,181 disclosed a radial flow NOx and CO converter for cars.
None of these approaches was completely satisfactory. While a moving bed reactor, perhaps similar to a moving bed cracking unit or a moving bed reformer, could certainly be fabricated to operate with a NOx conversion catalyst, the cost would be high.
The conventional approaches, use of large monolithic elements would work for a time, but require frequent shutdown for mechanical cleaning or replacement of the catalyst elements. This could not be tolerated in an FCC unit, where run lengths of 2-3 years must be achieved. Use of swing reactors, or oversized reactors, could perhaps overcome the short cycle problem, but would add to the cost.
A reliable and low cost way to treat fines laden streams for NOx removal is highly desirable. The system has to be mechanically simple, and ideally should operate with relatively low pressure drop. The ideal system should be designed to remove and replace some of the catalyst without shutting down the unit.
This application discloses that beds of solid, particulate catalyst could be used in a new way for selective catalytic reduction of NOx, to achieve some unexpected benefits. By using a bed of solid catalyst, and flowing gas through the bed at a sufficient velocity to expand it, causes at least limited fluidization and makes the catalyst bed self cleaning, as far as catalyst fines are concerned.
While the pressure drop through such a bed would be reasonably low, it would be higher than the pressure drop through a honeycomb. An improved reactor design was developed to permit low pressure drop SCR. This is a reactor design with "more surface area for gas entry than exist" achieved via a plurality of vertical inlets extending from under the bed well into the bed. Such a design permits particulate bed catalytic conversion of NOx containing flue gas with an unexpectedly low pressure drop. The expanded bed reactor also permits, should the need arise, some replacement of catalyst.